Network Working Group J. Arkko
Internet-Draft Ericsson
Intended status: Informational M. Townsley
Expires: April 25, 2011 Cisco
October 22, 2010
IPv4 Run-Out and IPv4-IPv6 Co-Existence Scenariosdraft-arkko-townsley-coexistence-06
Abstract
When IPv6 was designed, it was expected that the transition from IPv4
to IPv6 would occur more smoothly and expeditiously than experience
has revealed. The growth of the IPv4 Internet and predicted
depletion of the free pool of IPv4 address blocks on a foreseeable
horizon has highlighted an urgent need to revisit IPv6 deployment
models. This document provides an overview of deployment scenarios
with the goal of helping to understand what types of additional tools
the industry needs to assist in IPv4 and IPv6 co-existence and
transition.
This document was originally created as input to the Montreal co-
existence interim meeting in October 2008, which led to the
rechartering of the Behave and Softwire working groups to take on new
IPv4 and IPv6 coexistence work. This document is published as a
historical record of the thinking at the time, but hopefully also
helps understand the rationale behind current IETF tools for co-
existence and transition.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
Arkko & Townsley Expires April 25, 2011 [Page 1]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 25, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Arkko & Townsley Expires April 25, 2011 [Page 2]

Internet-Draft IPv4 and IPv6 Co-Existence October 20101. Introduction
This document was originally created as input to the Montreal co-
existence interim meeting in October 2008, which led to the
rechartering of the Behave and Softwire working groups to take on new
IPv4 and IPv6 coexistence work. This document is published as a
historical record of the thinking at the time, but hopefully also
helps understand the rationale behind current IETF tools for co-
existence and transition.
When IPv6 was designed, it was expected that IPv6 would be enabled,
in part or in whole, while continuing to run IPv4 side-by-side on the
same network nodes and hosts. This method of transition is referred
to as "Dual-Stack" [RFC4213] and has been the prevailing method
driving the specifications and available tools for IPv6 to date.
Experience has shown that large-scale deployment of IPv6 takes time,
effort, and significant investment. With IPv4 address pool depletion
on the foreseeable horizon [Huston.IPv4], network operators and
Internet Service Providers are being forced to consider network
designs that no longer assume the same level of access to unique
global IPv4 addresses. IPv6 alone does not alleviate this concern
given the basic assumption that all hosts and nodes will be Dual-
Stack until the eventual sunsetting of IPv4-only equipment. In
short, the time-frames for the growth of the IPv4 Internet, the
universal deployment of Dual-Stack IPv4 and IPv6, and the final
transition to an IPv6-dominant Internet are not in alignment with
what was originally expected.
While Dual-Stack remains the most well-understood approach to
deploying IPv6 today, current realities dictate a re-assessment of
the tools available for other deployment models that are likely to
emerge. In particular, the implications of deploying multiple layers
of IPv4 address translation need to be considered, as well as those
associated with translation between IPv4 and IPv6 which led to the
deprecation of [RFC2766] as detailed in [RFC4966]. This document
outlines some of the scenarios where these address and protocol
translation mechanisms could be useful, in addition to methods where
carrying IPv4 over IPv6 may be used to assist in transition to IPv6
and co-existence with IPv4. We purposefully avoid a description of
classic Dual-Stack methods, as well as IPv6 over IPv4 tunneling.
Instead, this document focuses on scenarios which are driving tools
we have historically not been developing standard solutions around.
It should be understood that the scenarios in this document represent
new deployment models and are intended to complement, not replace
existing ones. For instance, Dual-Stack continues to be the most
recommended deployment model. Note that Dual-Stack is not limited to
Arkko & Townsley Expires April 25, 2011 [Page 4]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
situations where all hosts can acquire public IPv4 addresses. A
common deployment scenario is running Dual-Stack on the IPv6 side
with public addresses, and on the IPv4 side with just one public
address and a traditional IPv4 NAT. Generally speaking, offering
native connectivity with both IP versions is preferred over the use
of translation or tunneling mechanisms when sufficient address space
is available.
2. Scenarios
This section identifies five deployment scenarios which we believe
have a significant level of near to medium term demand somewhere on
the globe. We will discuss these in the following sections, while
walking through a bit of the design space to get an understanding of
the types of tools that could be developed to solve each. In
particular, we want the reader to be consider what type of new
equipment must be introduced in the network and where for each
scenario, which nodes must be changed in some way, and which nodes
must work together in an interoperable manner via a new or existing
protocol.
The five scenarios are:
o Reaching the IPv4 Internet with less than one global IPv4 address
per subscriber or subscriber household available (Section 2.1).
o Running a large network needing more addresses than those
available in private RFC 1918 address space (Section 2.2).
o Running an IPv6-only network for operational simplicity as
compared to Dual-Stack, while still needing access to the global
IPv4 Internet for some, but not all, connectivity (Section 2.3).
o Reaching one or more privately addressed IPv4 only servers via
IPv6 (Section 2.4).
o Accessing IPv6-only servers from IPv4 only clients (Section 2.5).
Arkko & Townsley Expires April 25, 2011 [Page 5]

Internet-Draft IPv4 and IPv6 Co-Existence October 20102.1. Reaching the IPv4 Internet
+----+ +---------------+
IPv4 host(s)-----+ GW +------IPv4-------------| IPv4 Internet |
+----+ +---------------+
<---private v4--->NAT<--------------public v4----------------->
Figure 1: Accessing the IPv4 Internet today
Figure 1 shows a typical model for accessing the IPv4 Internet today,
with the gateway device implementing a Network Address and Port
Translation (NAPT, or more simply referred to in this document as
NAT). The NAT function serves a number of purposes, one of which is
to allow more hosts behind the gateway (GW) than there are IPv4
addresses presented to the Internet. This multiplexing of IP
addresses comes at great cost to the original end-to-end model of
Internet, but nonetheless is the dominant method of access today,
particularly to residential subscribers.
Taking the typical residential subscriber as an example, each
subscriber line is allocated one global IPv4 address for it to use
with as many devices as the NAT GW and local network can handle. As
IPv4 address space becomes more constrained and without substantial
movement to IPv6, it is expected that service providers will be
pressured to assign a single global IPv4 address to multiple
subscribers. Indeed, in some deployments this is already the case.
2.1.1. NAT444
When there is less than one address per subscriber at a given time,
address multiplexing must be performed at a location where visibility
to more than one subscriber can be realized. The most obvious place
for this is within the service provider network itself, requiring the
service provider to acquire and operate NAT equipment to allow
sharing of addresses across multiple subscribers. For deployments
where the GW is owned and operated by the customer, this becomes
operational overhead for the Internet Service Provider (ISP) that it
will no longer be able to rely on the customer and the seller of the
GW device for.
This new address translation node has been termed a "Carrier Grade
NAT", or CGN [I-D.nishitani-cgn]. The CGN's insertion into the ISP
network is shown in Figure 2.
Arkko & Townsley Expires April 25, 2011 [Page 6]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
+----+ +---+ +-------------+
IPv4 host(s)-----+ GW +------IPv4---------+CGN+--+IPv4 Internet|
+----+ +---+ +-------------+
<---private v4--->NAT<----private v4------>NAT<----public v4--->
Figure 2: Employing two NAT devices, NAT444
This solution approach is known as "NAT444" or "Double-NAT" and is
discussed further in [I-D.wing-nat-pt-replacement-comparison].
It is important to note that while multiple levels of multiplexing of
IPv4 addresses is occurring here, there is no aggregation of NAT
state between the GW and CGN. Every flow that is originated in the
subscriber home is represented as duplicate state in the GW and CGN.
For example, if there are 4 PCs in a subscriber home, each with 25
open TCP sessions, both the GW and CGN must track 100 sessions each
for that subscriber line.
NAT444 has the enticing property that it seems, at first glance, that
the CGN can be deployed without any change to the GW device or other
node in the network. While it is true that a GW which can accept a
lease for a global IPv4 address would very likely accept a translated
IPv4 address as well, the CGN is neither transparent to the GW or the
subscriber. In short, it is a very different service model to offer
a translated IPv4 address vs. a global IPv4 address to a customer.
While many things may continue to work in both environments, some
end-host applications may break, and GW port-mapping functionality
will likely cease to work reliably. Further, if addresses between
the subscriber network and service provider network overlap,
ambiguous routes in the GW could lead to misdirected or black-holed
traffic [I-D.shirasaki-isp-shared-addr]. Resolving this overlap
through allocation of new private address space is difficult, as many
existing devices rely on knowing what address ranges represent
private addresses [I-D.azinger-additional-private-ipv4-space-issues].
Network operations which had previously been tied to a single IPv4
address for a subscriber would need to be considered when deploying
NAT444 as well. These may include troubleshooting and OAM,
accounting, logs (including legal intercept), QoS functions, anti-
spoofing and security, backoffice systems, etc. Ironically, some of
these considerations overlap with the kinds of considerations one
needs to perform when deploying IPv6.
Consequences aside, NAT444 service is already being deployed in some
networks for residential broadband service. It is safe to assume
Arkko & Townsley Expires April 25, 2011 [Page 7]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
that this trend will likely continue in the face of tightening IPv4
address availability. The operational considerations of NAT444 need
to be well documented.
NAT444 assumes that the global IPv4 address offered to a residential
subscriber today will simply be replaced with a single translated
address. In order to try and circumvent performing NAT twice, and
since the address being offered is no longer a global address, a
service provider could begin offering a subnet of translated IPv4
addresses in hopes that the subscriber would route IPv4 in the GW
rather than NAT. The same would be true if the GW was known to be an
IP-unaware bridge. This makes assumptions on whether the ISP can
enforce policies, or even identify specific capabilities, of the GW.
Once we start opening the door to making changes at the GW, we have
increased the potential design space considerably. The next section
covers the same problem scenario of reaching the IPv4 Internet in the
face of IPv4 address depletion, but with the added wrinkle that the
GW can be updated or replaced along with the deployment of a CGN (or
CGN-like) node.
2.1.2. Distributed NAT
Increasingly, service providers offering "triple-play" services own
and manage a highly-functional GW in the subscriber home. These
managed GWs generally have rather tight integration with the service
provider network and applications. In these types of deployments, we
can begin to consider what other possibilities exist besides NAT444
by assuming cooperative functionality between the CGN and GW.
If the connection between the GW and CGN is a point-to-point link (a
common configuration between the GW and the "IP-Edge" in a number of
access architectures), NAT-like functionality may be "split" between
the GW and CGN rather than performing NAT444 as described in the
previous section.
one frac addr one public addr
+----+ +---+ +-------------+
IPv4 host(s)-----+ GW +-----p2p link------+CGN+--+IPv4 Internet|
+----+ +---+ +-------------+
<---private v4---> NAT <----public v4--->
(distributed,
over a p2p link)
Figure 3: Distributed-NAT service
Arkko & Townsley Expires April 25, 2011 [Page 8]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
In this approach, multiple GWs share a common public IPv4 address,
but with separate, non-overlapping, port ranges. Each such address/
port range pair is defined as a "fractional address". Each home
gateway can use the address as if it were its own public address,
except that only a limited port range is available to the gateway.
The CGN is aware of the port ranges, which may be assigned in
different ways, for instance during DHCP lease acquisition or
dynamically when ports are needed [I-D.despres-v6ops-apbp]. The CGN
directs traffic to the fractional address towards that subscriber's
GW device. This method has the advantage that the more complicated
aspects of the NAT function (Application Layer Gateways (ALGs), port-
mapping, etc.) remain in the GW, augmented only by the restricted
port-range allocated to the fractional address for that GW. The CGN
is then free to operate in a fairly stateless manner, forwarding
based on IP address and port ranges and not tracking any individual
flows from within the subscriber network. There are obvious scaling
benefits to this approach within the CGN node, with the tradeoff of
complexity in terms of the number of nodes and protocols that must
work together in an interoperable manner. Further, the GW is still
receiving a global IPv4 address, albeit only a "portion" of one in
terms of available port usage. There are still outstanding questions
in terms of how to handle protocols that run directly over IP and
cannot use the divided port number ranges, and handling of fragmented
packets, but the benefit is that we are no longer burdened by two
layers of NAT as in NAT444.
Not all access architectures provide a natural point to point link
between the GW and CGN to tie into. Further, the CGN may not be
incorporated into the IP Edge device in networks that do have point-
to-point links. For these cases, we can build our own point-to-point
link using a tunnel. A tunnel is essentially a point to point link
that we create when needed [I-D.ietf-intarea-tunnels]. This is
illustrated in Figure 4.
one frac addr one public addr
+----+ +---+ +-------------+
IPv4 host(s)-----+ GW +======tunnel=======+CGN+--+IPv4 Internet|
+----+ +---+ +-------------+
<---private v4---> NAT <----public v4--->
(distributed,
over a tunnel)
Figure 4: Point-to-point link created through a tunnel
Arkko & Townsley Expires April 25, 2011 [Page 9]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
Figure 4 is essentially the same as Figure 3, except the data link is
created with a tunnel. The tunnel could be created in any number of
ways depending on the underlying network.
At this point, we have used a tunnel or point-to-point link with
coordinated operation between the GW and CGN in order to keep most of
the NAT functionality in the GW.
Given the assumption of a point-to-point link between GW and CGN, the
CGN could perform the NAT function, allowing private, overlapping,
space to all subscribers. For example, each subscriber GW may be
assigned the same 10.0.0.0/8 address space (or all RFC 1918 [RFC1918]
space for that matter). The GW then becomes a simple "tunneling
router" and the CGN takes on the full NAT role. One can think of
this design as effectively a layer-3 VPN, but with Virtual-NAT tables
rather than Virtual-Routing tables.
2.1.3. Recommendation
This section dealt strictly with the problem of reaching the IPv4
Internet with limited public address space for each device in a
network. We explored combining NAT functions and tunnels between the
GW and CGN to obtain similar results with different design tradeoffs.
The methods presented can be summarized as:
a. Double-NAT (NAT444)
b. Single-NAT at CGN with a subnet and routing at the GW
c. Tunnel/link + Fractional IP (NAT at GW, port-routing at CGN)
d. Tunnel/link + Single NAT with overlapping RFC 1918 ("Virtual NAT"
tables and routing at the GW)
In all of the above, the GW could be logically moved into a single
host, potentially eliminating one level of NAT by that action alone.
As long as the hosts themselves need only a single IPv4 address,
methods b and d obviously are of little interest. This leaves
methods a and c as the more interesting methods in cases where there
is no analogous GW device (such as a campus network).
This document recommends the development of new guidelines and
specifications to address the above methods. Cases where the home
gateway both can and cannot be modified should be addressed.
Arkko & Townsley Expires April 25, 2011 [Page 10]

Internet-Draft IPv4 and IPv6 Co-Existence October 20102.2. Running out of IPv4 Private Address Space
In addition to public address space depletion, very large privately
addressed networks are reaching exhaustion of RFC 1918 space on local
networks as well. Very large service provider networks are prime
candidates for this. Private address space is used locally in ISPs
for a variety of things, including:
o control and management of service provider devices in subscriber
premises (cable modems, set-top boxes, and so on) and
o addressing the subscriber's NAT devices in a double NAT
arrangement, and
o "walled garden" data, voice, or video services.
Some providers deal with this problem by dividing their network into
parts, each on its own copy of the private space. However, this
limits the way services can be deployed and what management systems
can reach what devices. It is also operationally complicated in the
sense that the network operators have to understand which private
scope they are in.
Tunnels were used in the previous section to facilitate distribution
of a single global IPv4 address across multiple endpoints without
using NAT, or to allow overlapping address space to GWs or hosts
connected to a CGN. The kind of tunnel or link was not specified.
If the tunnel used carries IPv4 over IPv6, the portion of the IPv6
network traversed naturally need not be IPv4 capable, and need not
utilize IPv4 addresses, private or public, for the tunnel traffic to
traverse the network. This is shown in Figure 5.
IPv6-only network
+----+ +---+ +-------------+
IPv4 host--------+ GW +=======tunnel========+CGN+--+IPv4 Internet|
+----+ +---+ +-------------+
<---private v4----> <----- v4 over v6 -----> <---public v4---->
Figure 5: Running IPv4 over IPv6-only network
Each of the four approaches (a, b, c and d) from the Section 2.1
scenario could be applied here, and for brevity each iteration is not
specified in full here. The models are essentially the same, just
Arkko & Townsley Expires April 25, 2011 [Page 11]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
that the tunnel is over an IPv6 network and carries IPv4 traffic.
Note that while there are numerous solutions for carrying IPv6 over
IPv4, this reverse mode is somewhat of an exception (one notable
exception being the Softwire working group, as seen in [RFC4925]).
Once we have IPv6 to the GW (or host, if we consider the GW embedded
in the host), enabling IPv6 and IPv4 over the IPv6 tunnel allows for
Dual-Stack operation at the host or network behind the GW device.
This is depicted in Figure 6:
+----+ +-------------+
IPv6 host-----+ | +------------------+IPv6 Internet|
| +---IPv6-----+ +-------------+
Dual-Stack host-+ GW |
| | +---+ +-------------+
IPv4 host-----+ +===v4 over v6 tunnel====+CGN+--+IPv4 Internet|
+----+ +---+ +-------------+
<-----------private v4 (partially in tunnel)-->NAT<---public v4---->
<-----------------------------public v6---------------------------->
Figure 6: "Dual-Stack Lite" operation over an IPv6-only network
In [I-D.ietf-softwire-dual-stack-lite] this is referred to as "Dual-
Stack Lite" bowing to the fact that it is Dual-Stack at the gateway,
but not at the network. As introduced in Section 2.1, if the CGN
here is a full functioning NAT, hosts behind a Dual-Stack Lite
gateway can support IPv4-only and IPv6-enabled applications across an
IPv6-only network without provisioning a unique IPv4 addresses to
each gateway. In fact, every gateway may have the same address.
While the high-level problem space in this scenario is to alleviate
local usage of IPv4 addresses within a service provider network, the
solution direction identified with IPv6 has interesting operational
properties that should be pointed out. By tunneling IPv4 over IPv6
across the service provider network, the separate problems of
transition the service provider network to IPv6, deploying IPv6 to
subscribers, and continuing to provide IPv4 service can all be
decoupled. The service provider could deploy IPv6 internally, turn
off IPv4 internally, and still carry IPv4 traffic across the IPv6
core for end users. In the extreme case, all of that IPv4 traffic
need not be provisioned with different IPv4 addresses for each
endpoint as there is not IPv4 routing or forwarding within the
network. Thus, there are no issues with IPv4 renumbering, address
space allocation, etc. within the network itself.
Arkko & Townsley Expires April 25, 2011 [Page 12]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
It is recommended that the IETF develop tools to address this
scenario for both a host and GW. It is assumed that both endpoints
of the tunnel can be modified to support these new tools.
2.3. Enterprise IPv6 Only Networks
This scenario is about allowing an IPv6-only host or a host which has
no interfaces connected to an IPv4 network, to reach servers on the
IPv4 internet. This is an important scenario for what we sometimes
call "greenfield" deployments. One example is an enterprise network
that wishes to operate only IPv6 for operational simplicity, but
still wishes to reach the content in the IPv4 Internet. For
instance, a new office building may be provisioned with IPv6-only.
This is shown in Figure 7.
+----+ +-------------+
| +------------------+IPv6 Internet+
| | +-------------+
IPv6 host-----------------+ GW |
| | +-------------+
| +------------------+IPv4 Internet+
+----+ +-------------+
<-------------------------public v6----------------------------->
<-------public v6--------->NAT<----------public v4-------------->
Figure 7: Enterprise IPv6-only network
Other cases that have been mentioned include "greenfield" wireless
service provider networks and sensor networks. This bears a striking
resemblance to Section 2.2 as well, if one considers the service
provider network to simply be a very special kind of enterprise
network.
In the Section 2.2 scenario, we dipped into design space enough to
illustrate that the service provider was able to implement an IPv6-
only network to ease their addressing problems via tunneling. This
came at the cost of touching two devices on the edges of this
network; both the GW and the CGN have to support IPv6 and the
tunneling mechanism over IPv6. The greenfield enterprise scenario is
different from that one in the sense that there is only one place
that the enterprise can easily modify: the border between its network
and the IPv4 Internet. Obviously, the IPv4 Internet operates the way
it already does. But in addition, the hosts in the enterprise
network are commercially available devices, personal computers with
Arkko & Townsley Expires April 25, 2011 [Page 13]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
existing operating systems. While we consider in this scenario that
all of the devices on the network are "modern" Dual-Stack capable
devices, we do not want to have to rely upon kernel-level
modifications to these OSes. This restriction drives us to a "one
box" type of solution, where IPv6 can be translated into IPv4 to
reach the public Internet. This is one situation where new or
improved IETF specifications could have an effect to the user
experience in these networks. In fairness, it should be noted that
even a network-based solution will take time and effort to deploy.
This is essentially, again, a tradeoff between one new piece of
equipment in the network, or a cooperation between two.
One approach to deal with this environment is to provide an
application level proxy at the edge of the network (GW). For
instance, if the only application that needs to reach the IPv4
Internet is the web, then a HTTP/HTTPS proxy can easily convert
traffic from IPv6 into IPv4 on the outside.
Another more generic approach is to employ an IPv6 to IPv4 translator
device. This is discussed in
[I-D.wing-nat-pt-replacement-comparison]. NAT64 is an one example of
a translation scheme falling under this category
[I-D.ietf-behave-v6v4-framework] [I-D.ietf-behave-dns64]
[I-D.ietf-behave-v6v4-xlate] [I-D.ietf-behave-v6v4-xlate-stateful]
[I-D.ietf-behave-address-format].
Translation will in most cases have some negative consequences for
the end-to-end operation of Internet protocols. For instance, the
issues with Network Address Translation - Protocol Translation
(NAT-PT) [RFC2766] have been described in [RFC4966]. It is important
to note that the choice of translation solution and the assumptions
about the network where they are used impact the consequences. A
translator for the general case has a number of issues that a
translator for a more specific situation may not have at all.
It is recommended that the IETF develop tools to address this
scenario. These tools need to allow existing IPv6 hosts to operate
unchanged.
2.4. Reaching Private IPv4 Only Servers
This section discusses the specific problem of IPv4-only capable
server farms that no longer can be allocated a sufficient number of
public addresses. It is expected that for individual servers,
addresses are going to be available for a long time in a reasonably
easy manner. However, a large server farm may require a large enough
block of addresses that it is either not feasible to allocate one or
it becomes economically desirable to use the addresses for other
Arkko & Townsley Expires April 25, 2011 [Page 14]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
purposes.
Another use case for this scenario involves a service provider that
is capable of acquiring a sufficient number of IPv4 addresses, and
has already done so. However, the service provider also simply
wishes to start to offer an IPv6 service but without yet touching the
server farm by upgrading it to IPv6.
One option available in such a situation is to move those servers and
their clients to IPv6. However, moving to IPv6 is not just the cost
of the IPv6 connectivity, but the cost of moving the application
itself away to IPv6. So, in this case the server farm is IPv4 only,
there is an increasing cost for IPv4 connectivity, and an expensive
bill for moving server infrastructure to IPv6. What can be done?
If the clients are IPv4-only as well, the problem is a hard one, and
dealt with in more depth in Section 2.5. However, there are
important cases where large sets of clients are IPv6-capable. In
these cases it is possible to place the server farm in private IPv4
space and arrange some of gateway service from IPv6 to IPv4 to reach
the servers. This is shown in Figure 8.
+----+
IPv6 Host(s)-------(Internet)-----+ GW +------Private IPv4 Servers
+----+
<---------public v6--------------->NAT<------private v4---------->
Figure 8: Reaching servers in private IPv4 space
One approach to implement this is to use NAT64 to translate IPv6 into
private IPv4 addresses. The private IPv4 addresses are mapped into
IPv6 addresses within a known prefix(es). The GW at the edge of the
server farm is aware of the mapping, as is DNS. AAAA records for
each server name is given an IPv6 address that corresponds to the
mapped private IPv4 address. Thus, each privately addressed IPv4
server is given a public IPv6 presentation. No DNS application level
gateway (DNS-ALG) is needed in this case, contrary to what NAT-PT
required, for instance.
This is very similar to Section 2.3 where we typically think of a
small site with IPv6 needing to reach the public IPv4 Internet. The
difference here is that we assume not a small IPv6 site, but the
whole of the IPv6 Internet needing to reach a small IPv4 site. This
example was driven by the enterprise network with IPv4 servers, but
Arkko & Townsley Expires April 25, 2011 [Page 15]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
could be scaled down to the individual subscriber home level as well.
Here, the same technique could be used to, say, access an IPv4 webcam
in the home from the IPv6 internet. All that is needed is the
ability to update AAAA records appropriately, an IPv6 client (which
could use Teredo [RFC4380] or some other method to obtain IPv6
reachability), and the NAT64 mechanism. In this sense, this method
looks much like a "NAT/FW bypass" function.
An argument could be made that since the host is likely Dual-Stack,
existing port mapping services or NAT traversal techniques could be
used to reach the private space instead of IPv6. This would have to
be done anyway if the hosts are not all IPv6-capable or connected.
However, in the case that they are, the alternative techniques force
additional limitations on the use of port numbers. In the case of
IPv6 to IPv4 translation, the full port space would be available for
each server even in the private space.
It is recommended that the IETF develop tools to address this
scenario. These tools need to allow existing IPv4 servers to operate
unchanged.
2.5. Reaching IPv6 Only Servers
This scenario is predicted to become increasingly important as IPv4
global connectivity sufficient for supporting server-oriented content
becomes significantly more difficult to obtain than global IPv6
connectivity. Historically, the expectation has been that for
connectivity to IPv6-only devices, devices would either need to be
IPv6 connected, or Dual-Stack with the ability to setup an IPv6 over
IPv4 tunnel in order to access the IPv6 Internet. Many "modern"
device stacks have this capability, and for them this scenario does
not present a problem as long as a suitable gateway to terminate the
tunnel and route the IPv6 packets is available. But, for the server
operator, it may be a difficult proposition to leave all IPv4-only
devices without reachability. Thus, if a solution for IPv4-only
devices to reach IPv6-only servers were realizable, the benefits
would be clear. Not only could servers move directly to IPv6 without
trudging through a difficult Dual-Stack period, but they could do so
without risk of losing connectivity with the IPv4-only Internet.
Unfortunately, realizing this goal is complicated by the fact that
IPv4 to IPv6 is considered "hard" since of course IPv6 has a much
larger address space than IPv4. Thus, representing 128 bits in 32
bits is not possible, barring the use of techniques similar to NAT64,
which uses IPv6 addresses to represent IPv4 addresses as well.
The main questions about this scenario are about the timing and
priority. While the expectation that this scenario may be of
Arkko & Townsley Expires April 25, 2011 [Page 16]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
importance one day is readily acceptable, at time of this writing
there are little or no IPv6-only servers of importance beyond
contrived cases that the authors are aware of. The difficulty of
making a decision about this case is that, quite possibly, when there
is sufficient pressure on IPv4 in order to see IPv6-only servers, the
vast majority of hosts either have IPv6 connectivity, or the ability
to tunnel IPv6 over IPv4 one way or another.
This discussion makes assumptions about what is a "server" as well.
For the majority of applications seen on the IPv4 Internet to date,
this distinction has been more or less clear. This is perhaps in no
small part due to the overhead today in creating a truly end to end
application in the face of the fragmented addressing and reachability
brought on by the various NATs and firewalls employed today. This is
beginning to shift, however, as we see more and more pressure to
connect people to one another in an end-to-end fashion -- with peer-
to-peer techniques, for instance -- rather than simply content server
to client. Thus, if we consider an "IPv6-only server" as what we
classically consider as an "IPv4 server" today, there may not be a
lot of demand for this in the near future. However, with a more
distributed model of the Internet in mind there may be more
opportunities to employ IPv6-only "servers" that we would normally
extrapolate based on past experience with applications.
It is recommended that IETF addresses this scenario, though perhaps
with a slightly lower priority than the others. In any case, when
new tools are developed to support this, it should be obvious that we
cannot assume any support for updating legacy IPv4 hosts in order to
reach the IPv6-only servers.
3. Security Considerations
Security aspects of the individual solutions are discussed in more
depth elsewhere, for instance in [I-D.ietf-softwire-dual-stack-lite]
[I-D.ietf-behave-v6v4-framework] [I-D.ietf-behave-dns64]
[I-D.ietf-behave-v6v4-xlate] [I-D.ietf-behave-v6v4-xlate-stateful]
[I-D.wing-nat-pt-replacement-comparison] [RFC4966]. This document
highlights just three issues:
o Any type of translation may have an impact how certain protocols
can pass through. For example, IPsec needs support for NAT
traversal, and the proliferation of NATs implies an even higher
reliance on these mechanisms. It may also require additional
support for new types of translation.
o Some solutions have a need to modify results obtained from DNS.
This may have an impact on DNS Security, as discussed in
Arkko & Townsley Expires April 25, 2011 [Page 17]

Internet-Draft IPv4 and IPv6 Co-Existence October 2010
[RFC4966]. Minimization or even elimination of such problems is
essential, as discussed in [I-D.ietf-behave-dns64].
o Tunneling solutions have their own security issues, for instance
the need to secure tunnel endpoint discovery or to avoid opening
up denial-of-service or reflection vulnerabilities
[I-D.ietf-v6ops-tunnel-security-concerns].
4. IANA Considerations
This document has no actions for IANA.
5. Conclusions
The authors believe that the scenarios outlined in this document are
among the top of the list of those that should to be addressed by the
IETF community in short order. For each scenario, there are clearly
different solution approaches with implementation, operations and
deployment tradeoffs. Further, some approaches rely on existing or
well-understood technology, while some require new protocols and
changes to established network architecture. It is essential that
these tradeoffs be considered, understood by the community at large,
and in the end well-documented as part of the solution design.
After writing the initial version of this document, the Softwire
working group was rechartered to address Section 2.2 scenario with a
combination of existing tools (tunneling, IPv4 NATs) and some minor
new ones (DHCP options) [I-D.ietf-softwire-dual-stack-lite].
Similarly, the Behave working group was rechartered to address
scenarios from Section 2.3, Section 2.4, and Section 2.5. At the
time this document is being published, proposals to address scenarios
from Section 2.1 are still under consideration for new IETF work
items.
This document set out to list scenarios that are important for the
Internet community. While it introduces some design elements in
order to understand and discuss tradeoffs, it does not list detailed
requirements. In large part, the authors believe that exhaustive and
detailed requirements would not be helpful at the expense of
embarking on solutions given our current state of affairs. We do not
expect any of the solutions to be perfect when measured from all
vantage points. When looking for opportunities to deploy IPv6,
reaching for perfection too far could become its own demise if we are
not attentive to this. Our goal with this document is to support
development of tools to help minimize the tangible problems that we
are experiencing now, as well as those that we can best anticipate
Arkko & Townsley Expires April 25, 2011 [Page 18]